1. Field of the Invention
The present invention relates to a liquid crystal display device and its fabricating method, and more particularly, to a liquid crystal display device and its fabricating method that reduces the number of mask processes in forming a thin film transistor.
2. Description of the Related Art
As demands for the information display device and the portable information display media is growing, research on a light-weight thin film type flat panel display (FPD) is becoming a trend of industry threatening the related art cathode ray tube (CRT) display device. One type of FPDs is a liquid crystal display (LCD), which displays an image using the optical anisotropy of liquid crystal material, is actively employed in notebook computers and a desktop monitors because of its high quality resolution, color display, and picture quality.
The LCD includes a color filter substrate (a first substrate), an array substrate (a second substrate), and a liquid crystal layer formed between the color filter substrate and the array substrate. A thin film transistor (TFT) is generally used as a switching device of the LCD and an amorphous silicon thin film or a polycrystalline silicon thin film is used as a channel layer of the TFT.
A fabrication process of the LCD generally requires a plurality of masking processes (namely, a photolithography process) for fabrication of the array substrate including the TFT. Accordingly, reducing the number of masking processes is necessary to increase a productivity.
The structure of a general LCD will now be described with reference to FIG. 1. FIG. 1 is a plan view showing a portion of the array substrate of the related art LCD. An actual LCD has N×M pixels with N gate lines intersecting M data lines. In FIG. 1, only one pixel is shown in the drawing to simplify the explanation.
As shown, in FIG. 1, the array substrate 10 includes a pixel electrode 18 formed on a pixel region, gate lines 16 and data lines 17 arranged vertically and horizontally on the substrate 10 to define a pixel electrode, and a TFT (the switching device) formed at the intersection of the gate line 16 and the data line 17.
The TFT includes a gate electrode 21 connected to the gate line 16, a source electrode 22 connected to the data line 17, and a drain electrode 23 connected to the pixel electrode 18. The TFT further includes a first insulation film (not shown) and a second insulation film (not shown) for insulating the gate electrode 21 and the source/drain electrodes 22 and 23, and an active pattern 24 for forming a conductive channel between the source electrode 22 and the drain electrode 23. The gate electrode 21 supplies a gate voltage at the conductive channel.
The source electrode 22 is electrically connected to a source region of the active-pattern 24 and the drain electrode 23 is electrically connected to a drain region of the active pattern 24 through a plurality of first contact holes 40A defined through the first insulation film and the second insulation film. A third insulation film (not shown) having a second contact hole 40B is formed on the drain electrode 23 so that the drain electrode 23 and the pixel electrode 18 are electrically connected.
A process for fabricating the LCD as described above will now be explained with reference to FIGS. 2A to 2F. FIGS. 2A to 2F are sequential cross-sectional views showing a fabrication process of the LCD taken along line I-I of FIG. 1. The TFT in the related art uses polycrystalline silicon as a channel layer. With reference to FIG. 2A, an active pattern 24 made of a polycrystalline silicon thin film is formed on a substrate 10 using photolithography.
Next, as shown in FIG. 2B, a first insulation film 15A and a conductive metal material are disposed on the entire surface of the substrate 10 already provided with the active pattern 24. Thereafter, the conductive metal material is selectively patterned using photolithography, thereby forming a gate electrode 21 over the active pattern 24. The gate electrode 21 is insulated from the active pattern 24 by the gate insulation film 15A interposed therebetween.
Then, a high density impurity ion is injected into a desired portion of the active pattern 24. The gate electrode 21 is used as a mask to form p+ or n+ source/drain regions 24A and 24B. The source/drain regions 24A and 24B make an ohmic contact with source/drain electrodes. Subsequently, as shown in FIG. 2C, the second insulation film 15B is disposed on the entire surface of the substrate 10 already provided with the gate electrode 21, thereafter, a portion of the first and second insulation films 15A and 15B are removed to form a first contact hole 40A exposing a portion of the source/drain regions 24A and 24B.
Furthermore, as shown in FIG. 2D, a conductive metal material is disposed on the entire surface of the substrate 10 and then patterned by using photolithography process to form a source electrode 22 connected to the source region 24A and a drain electrode 23 connected to the drain region 24B through plurality of the first contact hole 40A. In this case, a portion of the conductive metal layer forming the source electrode 22 extends in a first direction to be connected with the data line 17.
As shown in FIG. 2E, after a third insulation film 15C is disposed on the entire surface of the substrate 10, a second contact hole 40B is formed to expose a portion of the drain electrode 23 by photolithography. Finally, as shown in FIG. 2F, a transparent conductive metal material is disposed on the entire surface of the substrate 10 already provided with the third insulation film 15C, and patterned by photolithography, thereby forming the pixel electrode 18 connected with the drain electrode 23 within the second contact hole 40B.
As described above, when fabricating the LCD, including the polycrystalline silicon TFT, a total of six photolithography processes are required to pattern the active pattern, the gate electrode, the first contact hole, the source/drain electrode, the second contact hole and the pixel electrode. The photolithography process is a series of processes to form a desired pattern on a thin film-deposited substrate, consisting of a plurality of processes including coating a photosensitive material, exposing the photosensitive material, and developing the photosensitive material. Accordingly, the plurality of photolithography processes contribute to a reduction in production yield and increase in TFT defects. Since a mask designed to form a pattern is costly, an, increase in the number of masks significantly increases the LCD fabrication cost.